Kaluza-Klein Induced Supersymmetry Breaking for Braneworlds in Type IIB Supergravity

We consider Z_2-symmetric braneworlds arising from 5-sphere compactifications with 5-form flux in type IIB supergravity. This Kaluza-Klein reduction produces a D=5 theory which supports 1/2-supersymmetric Z_2-symmetric domain-wall solutions. However, upon lifting such solutions back to D=10, one finds that supersymmetry is broken by 5-sphere Kaluza-Klein effects. This happens owing to the action on the Killing spinor of the Z_2 \subset SO(1,9) symmetry, which requires an orientation-reversing transformation in the 5-sphere directions together with the flip of the orbifold coordinate. We study the consequences of this supersymmetry breaking for the masses of fermion fluctuation modes about the brane background and find a natural two-scale hierarchy: some bulk modes have characteristic masses of order 1/L_5 but other modes more closely associated to the branes have an additional factor exp(-\rho/L_5), where L_5 is the AdS_5 length parameter and \rho is the orbifold size.